The invention relates to a carbon paste for self-calcining electrodes, in particular Soederberg anodes, refractory linings and connections for electrode joining points, having granulates of anthracite, graphite, petroleum coke, pitch coke, metallurgical coke and/or residual lining or electrode material as solid component and also a binder, and relates too to a process for manufacturing this carbon paste.
Refractory linings, self-calcining electrodes and connections for electrode joining points are usually made from carbon masses which are produced from calcined or graphitized solids and binders. Coal-tar pitch is widely used as a binder, as it usually exhibits a large dry or coke residue, as a rule 50-60 wt %, on calcining or graphitizing. Known from U.S. Pat. No. 3,261,892 is the production of preshaped electrodes in which powder-to-granular type carbon particles are sprayed with 5-15 volume percent liquid bitumen (with respect to the electrode volume), brought into a chosen shape and baked at about 300.degree. C. in an exothermic oxidation process.
British Pat. No. 1 434 824 describes a process for producing pre-shaped electrodes. Ground, calcined carbon fibers and a binder that can be coked are mixed together at a temperature at which the binder is liquid. Bitumen is mentioned there as one of the cokeable binders.
In the German standard DIN No. 55946 bitumen is defined as the dark, semi-solid to brittle, high molecular hydrocarbon mixture which can be melted, produced in the selective refining of mineral oils, also the fraction of natural asphalts that is soluble in carbon disulphide, and native paraffin and montan wax.
In none of these documents is reference made to health aspects.
Various investigations have shown that coal tar pitch contains polycyclic aromatic hydrocarbons which can escape in a gaseous form during production and/or use of refractory linings and self-calcining electrodes. The polycyclic aromatic hydrocarbons can be carcinogenic. The binder vapors evolved during the production or use of the carbon pastes of the kind mentioned hereinabove containing coal tar pitch as binder are therefore capable of being a health hazard for the industrial employee and for the environment in general.
In a number of west european countries and in the USA there already exist therefore regulations designed to protect the industrial employee from potentially hazardous binder vapors. Such regulations are expected in other countries in the near future.
The manufacturer is refractory furnace linings and electrodes or their users are now faced with the obligation to reduce the potentially hazardous polycyclic aromatic hydrocarbons to the legally specified limits using scrubbers or to render the vapors harmless in some other way.